The present invention generally relates to materials and processes for producing castings for high temperature applications, and particularly buckets for steam turbines intended to have operating temperatures that exceed 1300° F. (about 705° C.).
Components of steam turbines, such as nozzles (stationary blades) and buckets (rotating blades) of steam turbines, are typically formed of stainless steel, nickel, and cobalt-base alloys that exhibit desirable mechanical properties at typical steam turbine operating temperatures of about 1000° F. to about 1050° F. (about 538° C. to about 566° C.). Because the efficiency of a steam turbine plant is dependent on its operating temperature, there is a demand for components and particularly turbine buckets and nozzles that are capable of withstanding higher operating temperatures of 1300° F. (about 705° C.) and above. In particular, the development of next generation steam turbines capable of maximum operating temperatures of up to about 1400° F. (about 760° C.) are currently under consideration.
As the operating temperatures for steam turbine components increase, different alloy compositions and processing methods must be used to achieve a balance of mechanical, physical and environmental properties required for the applications. Steam turbine buckets capable of withstanding temperatures in excess of 1300° F. (about 705° C.) will require bucket alloys having substantially improved creep-rupture and stress relaxation capabilities compared to current steam turbine bucket alloys such as martensitic stainless steel Crucible 422, and compared to intermediate strength nickel-base alloys such as Waspaloy. In addition, suitable bucket alloys must also meet or exceed component yield strength requirements and resist environmental cracking and other types of degradation in steam, while also minimizing overall component cost.